Dissertations / Theses on the topic 'Ionospheric studies'

The first comprehensive study of equatorial- to mid-latitude ionospheric electric fields (plasma drifts) is presented, using extensive incoherent scatter radar measurements from Jicamarca, Arecibo, and Millstone Hill, and F-region ion drift meter data from the polar orbiting DE-2 satellite. Seasonal and solar cycle dependent empirical quiet-time electric field models from equatorial to mid latitudes are developed, which improve and extend existing climatological models. The signatures of electric field perturbations during geomagnetically disturbed periods, associated with changes in the high-latitude currents and the characteristics of storm-time dynamo electric fields driven by enhanced energy deposition into the high-latitude ionosphere, are studied. Analytical empirical models that describe these perturbation drifts are presented. The study provided conclusive evidence for the two basic components of ionospheric disturbance electric fields. It is shown that magnetospheric dynamo electric fields can penetrate with significant amplitudes into the equatorial- to mid-latitude ionosphere, but only for periods up to 1 hour, consistent with results from the Rice Convection Model. The storm-time wind-driven electric fields are proportional to the high-latitude energy input, vary with local time and latitude, and have largest magnitudes during nighttime. These perturbations affect differently the zonal and meridional electric field components. It is shown that equatorial zonal electric fields (vertical drifts) can be disturbed up to 30 hours after large enhancements in the high-latitude currents. These perturbation electric fields are associated with enhanced high-latitude energy deposition taking place predominantly between about 1-12 hours earlier and found to be in good agreement with the Blanc-Richmond disturbance dynamo model. A second class of perturbations occurs around midnight and in the dawn-noon sector with delays of about 18-30 hours between the equatorial- and the high-latitude disturbances , and maximizes during locally quiet geomagnetic times. The latitudinal variation of the meridional disturbance electric fields (zonal drifts) is also presented. It is shown that these perturbation electric fields are predominantly downward/equatorward at all latitudes and due to both prompt penetration and disturbance dynamo electric fields. These results are also generally consistent with predictions from global convection and disturbance dynamo models.

Travelling Ionospheric Disturbances (T.I.D.s) are perturbations of the plasma in the ionosphere caused by the passage of acoustic gravity waves. These oscillations fall into two distinct classes, labelled large scale (LS) and medium scale (MS), respectively. These TIDs have frequently been observed at high-latitude and their source of generation may be the magnetic activity and/or the weather system within the region. This thesis presents the analysis of TIDs observations taken over a eight year (1981- 1989) period by means of an advanced Incoherent Scatter radars (EISCAT radar system) at F-region high-latitude, employing CP-2 common programme. An eight year synoptic study of TID effects in electron density measured at various heights is presented. A similar investigation of the field aligned ion velocity has also been undertaken. These two data sets have been computed and the wave disturbance in the parameter quantified. Large scale disturbances occured more frequently than those of medium scale and both are observed most frequently during daytime and at the first equinox. There are two dominant azimuths for wave propagation southwest and northeast. A very clear example of wave activity was observed by EISCAT facility on April 25th, 1989 between 0800-1300 UT in the measurements of Ne, Te, Ti, and Vi, extending to hundreds of kilometres height in the ionosphere. The wave trace amplitude shows amplification with height. The relative phase difference between the TID amplitude in Ne/Ne, Te and Ti with Vi has been established. Comparative cases have been presented between the theoretical and the observed values to test Hooke's formula and Hines dispersion relation. It has been noticed that there was no correlation between the three-hourly Kp indices, the hourly AE indices or the international daily sunspot number Ri indices with the average values of the TIDs spectral power. Good correlation was found between the low cloud type occurrence and the MSTIDs main parameters. A method of estimating the horizontal distance of the wave source from the radar position is presented.

Parametric instability processes are thought to produce Stimulated Electromagnetic Emissions (SEE) during ionospheric heating experiments. The phenomenon is primarily attributed to plasma turbulence excited by the high frequency HF heater in the altitude region where the pump frequency ω₀ is near the plasma upper hybrid frequency ω_uh. In this study, parametric instability processes thought to produce SEE are studied using theoretical and electrostatic Particle-In-Cell PIC simulation models. The simulation plasma is driven with a uniform oscillating electric field directed nearly perpendicular to the background geomagnetic field {B} to consider interactions when ω_uh is near electron cyclotron harmonics nΩ_ce. The pump frequency and amplitude are varied to consider the effects on the simulation electric field power spectrum. In this study, theoretical predictions and numerical simulations are used to study the three-wave decay instability process thought to be responsible for the generation of the down-shifted sidebands, the downshifted peak DP and the downshifted maximum DM. In particular, the lower hybrid decay instability LHDI and the ion cyclotron decay instability ICDI are studied in detail. The theory is used to provide the angular regime, with respect to the direction perpendicular to the magnetic field, at which the sidebands develop as well as the frequency and wavenumber regimes of both the LHDI and the ICDI. The effect of the temperature ratio T_e/T_i for both instabilities is discussed. A comparison between the theoretical predictions, the simulation electric field power spectrum and the experimental observations are presented in this study. Time evolution of both the LHDI and the ICDI is also investigated. The theoretical predictions are also used to investigate the cascading of the LHDI and the ICDI. The spectra show consistencies with the experimental observations. A four-wave parametric decay instability process thought to be responsible for SEE broad up-shifted sideband spectral features is discussed as well. Many theoretical results are presented, in which the effect of stepping the heater frequency closer to the upper hybrid frequency on the angle of maximum growth θ_max, the growth rate γ and on both the frequency and wavenumber regimes of the four-wave process is investigated. The simulation electric field power spectrum showed a large amplitude up-shifted sideband and a much smaller amplitude down-shifted sideband, consistent with the experimental observations. Comparisons between the theoretical predictions, the simulation electric field power spectrum and the experimental observations are discussed in detail. The time evolution of the four-wave process is one important aspect that is also presented in this study. The development of density irregularities, cavities and particle heating is also analyzed and investigated in this study.
Ph. D.

Includes bibliographical references
Global Navigation Satellite Systems (GNSS) are used to provide information on position, time and velocity all over the world at any time of the day. Currently there are four operational GNSS and one of them is GPS (Global Positioning System) that is developed and maintained by U.S Department of Defence (DoD), which is widely used and accessible all over the world. The accuracy of the output or even the availability of the navigation system depends on current space weather conditions, which can cause random fluctuations of the phase and amplitude of the received signal, called scintillation. Interference of GNSS signals that are reflected and refracted from stationary objects on the ground, with signals that travel along a direct path via the ionosphere to the antenna, cause errors in the measured amplitude and phase. These errors are known as multipath errors and can lead to cycle slip and loss of lock on the satellite or degradation in the accuracy of position determination. High elevation cut off angles used for filtering GNSS signals, usually 15-30°, can reduce non-ionospheric interference due to multipath signals coming from the horizon. Since a fixed-elevation threshold does not take into consideration the surrounding physical environment of each GPS station, it can result in a significant loss of valuable data. Alternatively, if the fixed-elevation threshold is not high enough we run the risk of including multipath data in the analysis. In this project we characterized the multipath environment of the GPS Ionospheric Scintillation and TEC (Total Electron Content) Monitor (GISTM) receivers installed by SANSA (South African National Space Agency) at Gough Island (40:34oS and 9:88° W), Marion Island (46:87° S and 37:86° E), Hermanus (34:42° S and19:22° E) and SANAE IV (71:73° S and 2:2° W) by plotting azimuth-elevation maps of scintillation indices averaged over one year. The azimuth-elevation maps were used to identify objects that regularly scatter signals and cause high scintillation resulting from multipath effects. After identifying the multipath area from the azimuth-elevation map, an azimuth-dependent elevation threshold was developed using the MATLAB curve fitting tool. Using this method we are able to reduce the multi-path errors without losing important data. Using the azimuth-dependent elevation threshold typically gives 5 to 28% more useful data than using a 20° fixed-elevation threshold.

The TOPEX/Poseidon altimetry mission was developed as a NASA/CNES collaboration to provide accurate sea surface height (SSH) measurements. The TOPEX mission's altimeter is the first, and so far only, dual frequency system in space. The use of two frequencies allows a correction to be made for the radar pulse delay imposed by the Earth's ionosphere which would otherwise lead to an underestimation of SSH. Not only does TOPEX/Poseidon provide the most accurate SSH measurements yet from space, it also provides the first ever quasi-global measure of the integrated electron content (IEC) of the ionosphere. This thesis utilises TOPEX/Poseidon data in a combined study of both the oceans and the ionosphere. Firstly a study of the dependence of the IEC on geomagnetic disturbances, and the spatial coherence scale of the IEC is performed by comparison with the International Reference Ionosphere, an empirical ionospheric model. A systematic dependence of IEC with geomagnetic disturbance is found, and the first ever quasi-global maps of IEC spatial coherence distance are produced. This investigation may lead to an improvement in the accuracy of the model, and hence also that of single frequency altimeter systems, which must rely on such empirical models for their correction to the ionospheric delay. Secondly the tropical Pacific Ocean is studied, in particular the characteristics of large scale wave activity in relation to the devastating climatic/oceanic phenomena known as El Nino. Kelvin, Rossby and tropical instability waves are identified, and their interactions and possible mechanisms related to El Nino are investigated. Evidence for both western and eastern boundary reflections are presented, and their significance to the delayed oscillator mechanism is discussed. The study illustrates the necessity for highly accurate SSH measurements.

Stimulated electromagnetic emission (SEE) produced by interactions of high-power radio waves with the Earth's ionosphere is currently a topic of significant interest in ionospheric modification physics. SEE is believed to be produced by nonlinear wave-wave interactions involving the electromagnetic and electrostatic plasma waves in the altitude region where the pump wave frequency is near the upper hybrid resonance frequency. The most prominent upshifted feature in the SEE spectrum is the broad upshifted maximum (BUM). In this study, the instability processes thought to be responsible to the BUM spectra in the SEE experiments are discussed and analyzed using theoretical and electrostatic particle-in-cell (PIC) models. From characteristics of this feature, a four-wave parametric decay process has been studied as a viable mechanism for its production. The object is to (1) investigate the early time nonlinear development of the four-wave decay instability by using theoretical and numerical simulation models, (2) study the variation of the four-wave decay instability spectral features for a wide range of plasma and pump wave parameters, and (3) access its possible role in the production of the BUM spectral feature. Results of this investigation show that there is good agreement between predictions of the proposed theoretical model and the numerical simulation experiments. The simulation electric field power spectrum exhibits many of the important features of the experimental observations. The numerical simulation results show that consideration of the full nonlinear development of the four-wave parametric instability is crucial in providing insight into the asymmetric nature of the wave frequency spectrum observed during the experiments. The velocity-space ring-plasma instability, another generation mechanism for the BUM spectra, is studied using a theoretical model. The theoretical calculations show that the growth rate is larger in the region of the upper hybrid wave than that of the electron Bernstein wave. In addition, the effects of various plasma parameters are analyzed and it is predicted that the BUM should be more prominent with a hotter ring, at the direction perpendicular to the magnetic field, or in a closer region of cyclotron harmonic. A detailed comparison of the velocity space ring-plasma instability and the four-wave parametric process is presented where both the differences and the possible relations are discussed.
Ph. D.

Abstract Powerful radio waves can heat an electron gas via collisions between free electrons and neutral particles. Since the discovery of the Luxembourg effect in 1934, this effect is known to take place in the D-region ionosphere. According to theoretical models, the EISCAT Heating facility is capable of increasing the electron temperature by a factor of 5–10 in the D region, depending mostly on the electron density profile. Various indirect evidence for the existence of the D-region heating effect has been available, including successful modification of ionospheric conductivities and mesospheric chemistry. However, an experimental quantification of the electron temperature at its maximum in the heated D-region ionosphere has been missing. In particular, incoherent scatter (IS) radars should be able to observe directly plasma parameters, such as the electron temperature, although the heated D-region ionosphere is not a trivial target because of low electron density, and hence, small signal-to-noise ratio (SNR). In this thesis, Papers I and III present unique estimates for heated D-region electron temperatures based on IS measurements. It turned out that the theoretical predictions of the electron temperature generally agree with the few existing observations, at least at the altitudes of the maximum heating effect. Quite in contrast, when the D-region heating effect on the cosmic radio noise absorption was verified for the first time by the statistical data analysis presented in Paper II, the absorption enhancements due to heating were found to be an order of magnitude smaller than model results. The reason for this discrepancy remains still as open question, although one possible explanation is provided by the electron-temperature dependent ion chemistry, which was not taken into account in the modelling. The significance of the heating-induced ion chemistry effect in the D-region was investigated in Paper IV. There the heating-induced negative ion formation is proposed as a potential explanation for the observed modulation of Polar Mesosphere Winter Echo (PMWE) power.

Thesis (MEng (Electrical Engineering))--Cape Peninsula University of Technology, 2019
The goal of this project is to design, build and characterize a low power High Frequency (HF) transmitter. The transmitter will be installed and operated in Antarctica to establish a beacon at the South Pole station to be received by the Super Dual Auroral Radar Network (SuperDARN) radar installed at SANAE IV. The transmitter is specified and designed to transmit at 12.57 MHz (continuous wave) from the South Pole. It must achieve a frequency drift of 41.8_mHz or better. The transmitter must operate normally under extremely low temperature conditions down to -40°C. The transmitted wave will be refracted by the ionosphere and received by the SuperDARN radar at SANAE IV. The ground distance between the HF transmitter and the radar is approximately 2000 km. The goal of the experiment is to form a bi-static radar configuration in order to study the ionosphere, especially travelling ionospheric disturbances (TIDs), which are signatures of atmospheric gravity waves (AGWs). A 25 dBm transmitter prototype was developed using a GPS disciplined oscillator in order to achieve the frequency stability required for this project. The HF transmitter proved to conform to the power and frequency stability requirements both during propagation tests conducted between Hermanus (34.4241° S, 19.2247° E) and Pretoria (34.0558° S, 18.4589° E) in South Africa, as well as when the device was exposed to temperatures that ranged from +40°C to -45°C. Although the antenna design was beyond the scope of this project, various determinations and considerations are presented in the link budget analysis, which have been confirmed during field tests. Therefore, certain recommendations on the antenna design are given. Propagation in Antarctica is expected to differ from the field tests conducted due to the differences in density and dynamics of the polar ionosphere, compared to the mid-latitude ionosphere.

his dissertation serves to report on the novel use of a geodetic-grade, dual-frequency Global Positioning System (GPS) Ionospheric Scintillation and Total Electron Content Monitor (GISTM), in an attempt to identify instances of ionospheric scintillation over the South Atlantic Magnetic Anomaly (SAMA) while located aboard the moving polar research vessel SA Agulhas II. The SAMA is a region in the South Atlantic Ocean where the Earth’s magnetic field is weakest in relation to other regions at comparable latitudes, resulting in the precipitation of high-energy particles into the ionosphere during geomagnetic storms. Ionospheric scintillations are rapid fluctuations in the phase and amplitude of trans-ionospheric radio signals resulting from electron density variations along the ray path. As a result, spacebased navigation systems can encounter increased errors in position accuracy or complete loss of lock. These are risk factors for modern aircraft and ocean vessels which rely on access to accurate Position, Navigation and Timing (PNT) services to operate safely. In this research, only the radio signals from GPS satellites are specifically used to measure these fluctuations. Traditional scintillation measurements are done using dedicated dual-frequency GPS receivers at fixed terrestrial locations. Most of the SAMA lies beyond the reach of the land-based sensors. The South African National Space Agency (SANSA) operates several GISTM stations in Southern Africa, at Marion Island, Gough Island, and the SANAE-IV base in Antarctica. The NovAtel GSV4004B GPS Ionospheric Scintillation and Total Electron Content Monitor (GISTM) installed on board the SA Agulhas II in 2012 has enabled for the first time the terrestrial measurement of scintillation from within the SAMA region. In this project, the amplitude scintillation (S4) and phase scintillation (σφ) indices from 50 Hz L1 GPS signals recorded during the 2014 and 2015 voyages of the SA Agulhas II were analysed for the first time. The scintillation effects are characterised in terms of position and motion data, carrierto-noise-density ratio, number of satellites, and satellite lock time. The goal is to develop an understanding of the effect of motion on the quality of data recorded by the receiver. The roll angle thresholds for the SA Agulhas II are calculated and it is shown that multipath errors are unlikely to be experienced. Significant data challenges were identified stemming from the incorrect setup of the SA Agulhas II GISTM. Data from elevations below 10° were missing because of hard-coded limitations within the GISTM on-board software. The data underwent significant reprocessing before being used. Comparisons were done in-harbour and out at sea with data from the nearest stationary GISTM receivers. It was shown that the movement of the receiver induces significant noise in the data. The noise levels are proportional to the velocity of the ship. An attempt to filter out the noise was unsuccessful. The motion-induced noise in the ship data masked the presence of any potential scintillations. With the ability to detect scintillation compromised, it was decided that a comparison with a land-based receiver within the SAMA would be necessary. Only one identical GISTM receiver met these requirements, located on Gough Island, at 40°20’ 58.90" S, 9°52’ 49.35" W. Data was isolated from both the SA Agulhas II GISTM and Gough Island GISTM for a period where the separation between the two receiver locations was less than 100 km. The Symmetric-Horizontal disturbance index (SYM-H) was used to identify geomagnetic storm conditions. GPS visibility maps were used to identify any potential signal obstructions. No correlation could be seen between position error and the number of satellites locked due to the high number of GPS satellites available at all times. It was discovered that the high noise levels had no effect on the position accuracy of the moving receiver, but that rapid changes in the instantaneous velocity coincided with peaks in the position error. No scintillation events were identified using the SA Agulhas II GISTM as a result of masking by the noise, however, the Gough Island GISTM data showed that no scintillation events occurred during the period in question anyway. Wind was identified as a potential contributing factor to the motion noise effect. This study provided justification for the purchase and installation of a newly developed motion-compensated GISTM receiver on board the SA Agulhas II, running off the same antenna and thus the same received signals. These data sets can be used for a direct receiver comparison in future work.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007.
Includes bibliographical references (p. 97-100).
We report on observations of a series of highly-structured ionospheric plasma turbulence over Arecibo on the nights of 22/23 and 23/24 July, 2006. Incoherent scatter measurements by Arecibo radar, airglow measurements using MIT PSFC's all-sky imaging system (ASIS), together with TEC measurements from GPS satellite network provide well-integrated diagnostics of turbulent plasma conditions. Two kinds of turbulent structures were seen as slanted stripes and filaments/quasi-periodic echoes on the range-time-intensity (RTI) plots of radar measurements. Detailed analyses of radar, airglow, and GPS data allow us to determine the drift velocity/direction, the orientation/geometry, and the scale lengths of these plasma turbulence structures. They are large plasma sheets with tens of kilometer scale lengths, moving in the form of traveling ionospheric disturbances (TIDs) southward within the meridional plane or westward in zonal plane at tens of meter per second. The signatures of observed TIDs indicate that they were triggered by internal gravity waves that had reached the altitudes of ionospheric F region. All possible sources producing gravity waves have been examined. We rule out solar/geomagnetic conditions which were quiet, and the atmospheric weather anomalies which were absent, during the period of time for our experiments. It is found that the heat wave fronts, which occurred in US, were plausible sources of free energy generating intense gravity waves and triggering large plasma turbulence over Arecibo. In other words, anomalous heat sources can be responsible for the occurrence of intense space plasma turbulence all over the world. The reported research suggests that global warming may affect the space weather conditions significantly. Further GPS data analysis is outlined as our future efforts to verify some predictions based on the current research outcomes.
(cont.) Simulation experiments can be conducted at Gakona, Alaska using the powerful high-frequency active auroral research programs (HAARP) heating facility, to generate gravity waves for the controlled study of concerned intriguing phenomenon.
by Rezy Pradipta.
S.M.

Ionospheric disturbance electric fields of magnetospheric origin play an important role in determining the global morphology and dynamics of the ionosphere of the Earth. In this work, we present a number of numerical simulations of the transient electric fields in the middle and inner magnetosphere and the ionosphere equatorward of the auroral zone caused by idealized changes in the magnetospheric driving parameters. For these studies, we u se the Rice Convection Model (RCM), a large computer code of the magnetosphere-ionosphere coupling which consistently computes the electric fields, currents, and plasma densities in the magnetosphere and the electric field and currents in the ionosphere in the quasi-static slow-flow approximation. We made substantial upgrades to the code, which include a module computing realistic solar EDY-produced ionospheric conductances and a new potential solver. Our upgraded version of the RCM also includes a time - varying magnetospheric magnetic field and a self-consistently estimated auroral zone. We first discuss numerical problems encountered in modeling electrodynamics of convection with a time-varying magnetic field, realistic ionospheric conductances, and a self-consistent auroral zone, and our solutions to those difficulties. We then present a number of "computer experiments" with the new version of the RCM with idealized changes in the magnetospheric parameters such as sudden changes in the cross polar cap potential drop, magnetic field reconfiguration corresponding to the overall changes in the high-latitude convection, as well as rotations of the electric field on the polar cap boundary. Prompt penetration ionospheric electric fields simulated with the upgraded RCM are shown to be consistent with the previous simulations. The new simulations and their results are discussed in the context of (1) possible contribution to the variability of the ionospheric electric fields, and (2) role of time-varying magnetic field on the characteristic lifetimes of prompt penetration electric fields at subauroral, middle, and low latitudes.

Abstract This thesis studies the ionospheres of the Earth and Mars by combining the observations of versatile instruments providing information on various aspects of the planetary environments. The work on the terrestrial ionosphere focuses particularly on solar wind–magnetosphere–ionosphere coupling, while the work on the Martian ionosphere is based on the development of a new approach to analyse radio-occultation data to retrieve the atmospheric and ionospheric profiles. In the Earth's ionosphere, two papers study the effects of solar wind high-speed streams on the ionospheric F-region peak electron density and on cosmic noise absorption resulting from the precipitation of energetic (>30 keV) electrons into the D region. For the first paper, a modified version of the superposed epoch analysis method, called phase-locked superposed epoch analysis method, was developed. The main finding is that a depletion near the F-region peak takes place in the afternoon and evening sectors during high-speed stream events. This could be explained by an increase in the electron loss rate as a consequence of ion-neutral frictional heating, which enhances the ion temperature and leads to neutral atmosphere expansion. In addition, dayside and post-midnight F-peak electron density increases are observed, probably related to soft particle precipitation. The second study reveals that cosmic noise absorption occurs during up to 4 days after the arrival of a high-speed stream, as substorm-injected energetic electrons precipitate in the midnight to early-afternoon ionosphere, principally at auroral latitudes. A third study reports for the first time observations of a modulation of cosmic noise absorption at periods near 10 s, associated with pulsating aurora. This suggests that the energetic component of the precipitating ux is modulated consistently with the auroral (1–10 keV) energies. At Mars, radio-occultation experiments have been performed by the Mars Express spacecraft since 2004. In this thesis, a new data analysis approach is developed, based on the numerical simulation of radio wave propagation through modelled Martian atmosphere and ionosphere. This approach enables one to overcome limitations inherent in the classical inversion method which has been in use so far. It also gives access to new parameters such as ion density profiles. The new method is tested by analysing the data from two radio-occultation experiments
Tiivistelmä Tämä väitöskirja tutkii Maapallon ja Marsin ionosfäärejä yhdistämällä useiden eri instrumenttien havaintoja, joilla saadaan tietoa planeettojen ympäristöistä. Maapallon ionosfääriä koskeva työ tutkii aurinkotuuli–magnetosfääri–ionosfäärikytkentää, kun taas Marsin ionosfääriä koskevan työn tavoite on uuden radio-okkultaatiomittauksen data-analyysimenetelmän kehittäminen, joka tuottaa ilmakehän ja ionosfäärin profiileja. Maan ionosfäärin tapauksessa yhdessä julkaisussa tutkitaan nopeiden aurinkotuulivirtausten vaikutuksia F-kerroksen elektronitiheyteen ja toisessa julkaisussa tutkitaan energeettisten (>30 keV) elektronien sateesta johtuvaa kosmisen radiokohinan absorptiota D-kerroksessa. Ensimmäisessä julkaisussa on kehitetty uusi versio data-analyysimenetelmästä, jota kutsutaan vaihelukituksi epookkien superpositiomenetelmäksi. Julkaisun päätulos on, että nopeiden aurinkotuulivirtausten aikana F-kerroksen maksimielektronitiheys pienenee iltapäivän ja illan sektoreilla. Tämä voidaan selittää johtuvan siitä, että ioni-neutraalitörmäysten synnyttämä kitkalämpö kasvattaa ionilämpötilaa ja aiheuttaa lisäksi ilmakehän laajenemisen. Molemmat prosessit kasvattavat elektronien häviönopeutta. F-kerroksen elektronitiheysmaksimi puolestaan kasvaa sektorilla, joka ulottuu keskiyöstä aamun kautta keskipäivään, ja tämä johtuu matalaenergeettisestä elektronisateesta. Toisessa julkaisussa havaitaan, että lisääntynyt kosmisen radiokohinan absorptio kestää jopa neljä päivää nopean aurinkotuulivirtauksen saavuttua Maan kohdalle. Tämä johtuu siitä, että alimyrskyitse injektoidut energeettiset elektronit satavat keskiyön ja aamun ionosfääriin, pääasiassa revontuliovaalin alueella. Kolmas julkaisu raportoi ensimmäistä kertaa havainnon sykkiviin revontuliin liittyvästä kosmisen radiokohinan absorptiosta n. 10 s jaksollisuudella. Tämä osoittaa, että elektronivuon energeettinen komponentti on moduloitu samalla jaksollisuudella kuin revontulielektronien energiat (1–10 keV). Marsissa on tehty radio-okkultaatiomittauksia vuodesta 2004 saakka Mars Express -luotaimen avulla. Vaitoskirjassa on kehitetty uusi datan analyysimenetelmä, joka perustuu numeeriseen simulointiin radioaaltojen etenemisestä Marsin ilmakehässä ja ionosfäärissä. Tämän lähestymistavan avulla vältetään tähän asti käytetyn klassisen inversiomenetelmän rajoitukset. Lisäksi menetelmä tuottaa uusia parametrejä kuten ionitiheysprofiileja. Uutta menetelmää testattiin tulkiten kahden radio-okkultaatiomittauksen aineistoa
Résumé Le travail présenté dans ce manuscrit de thèse s'articule autour de l'étude des ionosphères terrestre et martienne. Une approche multi-instrumentale est adoptée afin de combiner des observations permettant de mettre en perspective des manifestations de phénomènes physiques de natures différentes mais intervenant dans un même contexte global. Le travail doctoral comporte également un volet modélisation. Le manuscrit de thèse consiste en une partie introductrice à laquelle sont adossées cinq publications dans des revues scientifiques à comité de lecture. La partie introductrice de ce manuscrit de thèse a pour objectif de présenter le contexte scientifique sur lequel est basé le travail doctoral. Un premier chapitre passe en revue les principaux aspects théoriques dans lesquels s'inscrivent les études dont les résultats sont publiés dans les cinq articles. Les atmosphères et ionosphères de la Terre et de Mars y sont succinctement décrites, de même que les interactions entre ces planètes et le vent solaire, comprenant notamment la formation de magnétosphères. Les deux chapitres suivants présentent les instruments dont sont issues les données utilisées dans ce travail doctoral ainsi que les méthodes d'analyse des données. Le quatrième chapitre résume les principaux résultats obtenus autour des trois grandes thématiques abordées au cours de cette thèse. Enfin, des pistes quant à la continuation potentielle du travail présenté dans ce manuscrit de thèse sont évoquées en conclusion. Le premier article porte sur une étude statistique des effets des courants de vent solaire rapide sur la région F de l'ionosphère aurorale. Il s'appuie sur des données mesurées par l'ionosonde de Sodankylä entre 2006 et 2008. Au cours de cette période, 95 événements associés à des courants de vent solaire rapide ont été sélectionnés, et la réponse de l'ionosphère au-dessus de Sodankylä a été étudiée à partir des fréquences critiques des régions E et F de l'ionosphère, qui donnent la valeur du pic de concentration électronique dans ces deux régions. Pour cela, une version modifiée de la méthode des époques superposées a été développée, appelée “méthode des époques superposées avec verrouillage de phase”. Une augmentation du pic de concentration des régions E et F est observée du côté nuit et le matin, en lien avec une activité aurorale accrue, tandis qu'une déplétion de la région F est révélée aux temps magnétiques locaux situés entre 12 h et 23 h. Une estimation des effets d'une possible modification de l'équilibre photo-chimique résultant d'un accroissement du chauffage issu de la friction entre les ions et les éléments neutres est proposée. Le deuxième article s'intéresse aux précipitations énergétiques dans l'ionosphère aurorale durant ces mêmes 95 événements, en étudiant l'absorption du bruit cosmique qui en résulte. Il apparaît que les événements au cours desquels le vent solaire demeure rapide pendant plusieurs jours produisent davantage de précipitations énergétiques, qui peuvent atteindre les latitudes subaurorales. Par ailleurs, trois types de précipitations énergétiques sont étudiés séparément, selon qu'elles sont associées avec des signatures de sous-orage magnétique, avec des pulsations géomagnétiques, ou ni l'un ni l'autre. Les deux premiers types de précipitations semblent liés. En effet, l'analyse des données suggère que les électrons énergétiques sont injectés dans la magnétosphère interne durant les sous-orages. Tandis qu'une partie d'entre eux précipitent immédiatement du côté nuit, d'autres dérivent vers le côté matin, où ils subissent des interactions avec des ondes électromagnétiques de type siffleur (whistler en anglais), qui peuvent être modulées par des pulsations géomagnétiques, menant à leur précipitation. Le troisième article présente pour la première fois l'observation de signatures d'aurore pulsante dans des données d'absorption du bruit cosmique. Ces signatures sont consistantes avec les pulsations observables dans l'émission aurorale, et semblent indiquer une modulation cohérente des composantes aurorale (1–10 keV) et énergétique (> 30 keV) du spectre des précipitations électroniques au cours d'une aurore pulsante. Le quatrième article introduit une nouvelle méthode proposée pour analyser les données de radio-occultation mesurées par la sonde Mars Express. Cette approche vise à contourner des difficultés posées par les hypothèses fortes nécessaires à la mise en œuvre de la méthode classique d'inversion, notamment celle d'un environnement martien à symétrie sphérique — qui n'est pas acceptable lors de sondages proches du terminateur jour-nuit. La nouvelle méthode est basée sur la modélisation de l'atmosphère et de l'ionosphère de Mars, et sur la simulation de la propagation des ondes radio entre la station sol sur Terre et Mars Express lors d'une expérience de radio-occultation. Les paramètres libres contrôlant les profils atmosphériques et ionosphériques sont ajustés afin que la simulation reproduise le plus fidèlement possible les mesures. Le cinquième article est une réponse à un commentaire sur l'article précédent. Il vise d'une part à répondre aux critiques émises sur la méthode développée, montrant que celles-ci n'en remettent en cause ni la validité ni la pertinence, et d'autre part à y apporter quelques améliorations

Abstract The general aim of the present study is to gain insight into physical mechanisms of some auroral forms on the basis of multi-instrumental measurements (satellites, rockets and ground-based magnetic and riometer instruments) in the vicinity of the auroras observed by ground-based all-sky cameras. One part of this work is related to the Auroral Turbulence II sounding rocket experiment. It was launched on February 11th, 1997, at 08:36 UT from Poker Flat Research Range, Alaska, into a moderately active auroral region after a substorm onset. This unique three-payload rocket experiment contained both electric and magnetic in the evening sector (21 MLT), auroral forms at the substorm recovery were investigated, providing details of the quiet and disturbed auroral densities and DC electric patches propagating along them like a luminosity wave. Those evening auroral patches and associated electric fields formed a 200-km spatially-periodic structure along the arc, which propagated westward at a velocity of 3 km s-1. The other part of this study describes ground signatures of dynamic substorm features observed by the IRIS imaging riometer, magnetometers and all-sky camera during late evening hours. The magnetometer data were consistent with the motion of upward data are used to estimate the intensity of FAC associated with these local current-carrying the excitation of the low-frequency turbulence in the upper ionosphere. As a result, a quasi-oscillating regime of anomalous resistivity on the auroral field lines can give rise to the burst-like electron acceleration responsible for simultaneously observed auroral forms and bursts of Pi1B pulsations.

The electron Pedersen conductivity instability (EPCI) was proposed by Dimant and Sudan (1995a, 1995b, 1995c) as an extension to accepted Farley-Buneman instability theory and was predicted to give rise to D-region radar echoes. Theoretical modelling of its growth rate and threshold velocity for varying flow and aspect angles is presented, together with evidence for a two-layer structure where the growth rate is maximised. Backscatter parameters obtained by two coherent radar systems, CUTLASS and STARE, are presented for two case studies where the data are consistent with an electrojet flow whose velocity exceeds the threshold value for EPCI excitation. Backscatter parameters obtained from artificially generated FAIs for spatial sweeping and preconditioning experiments are also presented. Varying the illumination time of part of the heated patch affects the CUTLASS backscatter power corresponding to that patch. The variation in backscatter over CUTLASS range gates, for a heater beam with varying pointing direction, is shown to agree closely with the expected results obtained by modelling the heater beam intensity. It is shown that the CUTLASS backscatter power, for a given heater power, is dependent upon whether the ionosphere has previously been excited at a higher heater power. A new longer-lag mode was run on CUTLASS for the October 1999 heating campaign and the ACF decorrelation and backscatter power decay time constants obtained from data collected when this mode was running were different by an order of magnitude. Turbulence characteristics were obtained from the artificial irregularity distribution. These are compared to a study preferred by Villain et al. (1996) and the results are consistent with an artificial irregularity distribution that remained correlated for longer times than naturally occurring irregularity distributions.

Ionospheric scintillations can cause significant amplitude and/or phase fluctuations of GNSS signals. This work presents analysis results of scintillation effects on the new GPS L5 signal based on data collected using a real-time scintillation monitoring and data collection system at HAARP, Alaska. The data collection setup includes a custom narrow band front end that collects GPS L1, L2 IF samples and two reconfigurable USRP2 based RF front ends to collect wideband GPS L5 and GLONASS L1 and L2 signals. The results confirm that scintillation has a stronger impact on GPS L2 and L5 signals than on the L1 signal. Our preliminary results also show that carrier phase and amplitude scintillations on each signal are highly correlated. The amplitude and carrier phase scintillation are also correlated among the three signals. In this study, a multi-constellation multi-band GNSS software receiver has been developed based on USRP2, a general purpose radio platform. The C++ class-based software receiver were developed to process the IF data for GPS L1, L2C, and L5 and GLONASS L1 and L2 signals collected by the USRP2 front end. The front end performance is evaluated against the outputs of a high end custom front end driven by the same local oscillator and two commercial receivers, all using the same real signal sources. These results demonstrate that the USRP2 is a suitable front end for applications, such as ionosphere scintillation studies. Another major contribution of this work is the implementation of a Vector tracking loop (VTL) for robust carrier tracking. The VTL is developed based on the extended Kalman filter (EKF) with adaptive covariance matrices. Both scalar tracking loop (STL) and VTL are implemented. Once an error in the scalar loop is detected, the results from the VTL are used to assist the STL. The performance of the VTL is compared with the traditional STL with three different data sets: raw GPS RF data with short signal outages, RF data with strong scintillation impacts collected during the last solar maximum, and high dynamic data with long interval signal outages from a GPS simulator. The results confirm the performance improvement of the VTL over scintillation impacts and show that the VTL can maintain signal lock during long intervals of signal outage if the satellite ephemerides are available and the pseudorange estimation is within one code chip accuracy. The dynamic performance improvement of the VTL is verified as well. The results show the potential of robust tracking based on VTL during scintillation and interference.
Ph. D.

Optical measurements of small-scale auroral structures are here combined with spectrographic data in order to study the relationship between auroral morphology and the energy characteristics of the precipitating population. It is shown that rayed auroral structures are associated with precipitating electrons with a broad range in energy, including a significant population at energies of around 100 eV. In comparison, observations of fast-moving auroral arc elements are shown to result from precipitation energy distributions peaking at several keV with a very small low-energy component. This spectrographic information feeds directly into the study of naturally enhanced ion-acoustic lines, or NEIALs, which have been observed by incoherent-scatter radars at high-latitudes. It has been proposed that these radar enhancements result from natural plasma instability, causing the generation of ion-acoustic waves through the decay of unstable Langmuir waves, themselves driven by low-energy electron streams. Using multi-spectral imaging in combination with radar observations, a direct link is shown between ion-acoustic wave enhancements and precipitating electrons at 100 eV energies. Wave enhancements at the radar wavevector which are three orders of magnitude above the thermal level, are successfully modelled using the Langmuir decay interpretation for the time of observation. Electron populations with a broad energy range are thought to result from Alfv´enic acceleration mechanisms, which play an important role in the generation of smallscale auroral structure. With the recent advancements in multi-spectral imaging, it is now possible to resolve auroral filaments of a few hundred meters width. An interferometric imaging capability is under development for the EISCAT Svalbard Radar system, in order to resolve scattering wave structures on similar spatial scales within the radar beam. A technique is demonstrated by which to calibrate the position of coherent echoes detected by the interferometer. This will be of great use in clarifying the role of precipitating electron beams in turbulent plasma processes on small scales.

Simulations of the Martian upper atmosphere have been conducted with ‘MarTIM’, University College London’s Martian thermosphere and ionosphere general circulation model (GCM). MarTIM, a finite difference model, solves the coupled non-linear Navier-Stokes equations of continuity and momentum as well as an energy equation with calculations conducted on a fixed co-rotating grid of variable size in the pressure coordinate system. From its lower boundary of 0.883 Pa (~60 km) to its upper boundary of 9.9×10−8 Pa (~200−350 km), it evaluates the main sources of solar forcing (EUV/UV and IR absorption) while self-consistently determining the composition of four of the major gas species, CO2, N2, CO and O. These four major gases are mutually diffused throughout the model in a typical run. Development of MarTIM includes a consideration of the importance of neutral species diffusion and advection on the thermodynamics of the modelled Martian atmosphere. The influence on the modelled atmosphere of including additional neutral species is investigated. Next, a new infrared heating parameterization has been introduced from background research of detailed non-LTE modelling. This has allowed MarTIM to study thermospheric polar warming features as found in Mars Odyssey accelerometer data. MarTIM’s lower boundary is coupled to the Mars Climate Database (MCD v4.3) developed by the University of Oxford, the Open University and Laboratoire de Météorologie Dynamique. This database of GCM results provides MarTIM a physically self-consistent lower boundary derived from multiple runs of the aforementioned circulation models. Consequently the effects of dust storms, non-migrating tides and the influence of Martian topography are studied by prescription of MarTIM’s lower boundary. MarTIM is also compared against density and temperature measurements derived from SPICAM stellar occultation profiles. Lastly, a new ionospheric code has been developed through collaboration with Laboratoire de Planétologie de Grenoble. This has provided a more sophisticated ionosphere model that solves a one-dimensional kinetic Boltzmann transport equation for the suprathermal population of electrons present in the Martian ionosphere. MarTIM can now self-consistently describe an ionosphere produced by both primary (photoionisation) and secondary ionisation (suprathermal electron propagation). This new ionospheric model has been used to study the variation in secondary ionization efficiency (ratio of secondary to primary ion production) through a large range of seasonal and solar conditions.

博士
國立成功大學
物理學系碩博士班
100
The purpose of this dissertation is to understand the phenomena leading to substorm onset and explore the relation between the observations in the near-Earth magnetosphere and in the ionosphere. In particular, we present two observational studies, which provide new insights to the substorm onset scenario. Firstly, we study thoroughly a specific substorm event using synoptic observations of the THEMIS ground-based All-Sky-Imagers, the ISUAL CCD Imager aboard the FORMOSAT-2 satellite, the ground-based magnetometers, and the geosynchronous satellites. In particular, we investigated the behavior of substorm auroral arcs and ground low-frequency fluctuations, in terms of the bead-like arc brightening spot structures of the substorm arcs and their azimuthal mode number, and the concurrent behavior of the arc intensity, the westward electroject intensity, and the ground Pi2 pulsation amplitude. The Pi2 pulsation is found to be related to the substorm auroral arc formation. We discuss the physical implications of these observed phenomena. Secondly, we investigated the statistical correlation between the wave-like brightening spot structure of the auroral arcs and the unstable Pi2 pulsations observed in the near-Earth plasma sheet by THEMIS satellites. We found that the azimuthal mode number of the wave-like substorm arcs decreases with increasing geomagnetic latitude of the substorm arc location. We also found that the propagation velocity of the wave-like arc bright spots is linearly correlated with the propagation velocity of the unstable Pi2 δBy pulsations in the plasma sheet. Finally, we proposed that the kinetic ballooning instability with high azimuthal mode number can explain the observed substorm onset phenomena.